As is now well known, the use of composite material in the construction of military and commercial aircraft is widespread and increasing. The most commonly used composites consist of a polymer matrix reinforced with fibers of such materials as carbon, glass or Kevlar.
In many cases, relatively thin panels are used. At the present time, panels 0.088" to 0.189" in thickness appear to be common, although thicker and thinner parts are also used.
To fasten such panels together, adhesive bonding is used wherever possible. When adhesive bonding alone is judged insufficient, fasteners are used in addition. When panels must be removed for service or other purposes, fasteners are used without any adhesive. Because many of the composite sheet thickness are not very different from the metal sheet thickness which have been used in aircraft construction for many years, many of the same metal fasteners which have been used and proven to be reliable in metal sheet can be used in composite sheet.
An example of such a fastener is the metal threaded shear pin and collar, hundreds of millions of which have been used in the aircraft industry over the past twenty years. This fastener looks like a nut and bolt, but is designed with a head and a threaded collar of minimum size to save weight. These size reductions are possible because shear pin fasteners are designed to carry mainly shear loads because joints in thin sheet seldom exert high tensile loadings on the fasteners installed in them.
A frequently used such shear pin for fastening composite is made from 6AL4V titanium, and a commonly used threaded collar or nut is made from A286 stainless steel or 6AL4V titanium. A much lighter nut made from aluminum would enable the full shear strength of the joint to be developed in most cases, but the corrosion rate of the aluminum is so seriously increased when in contact with carbon fiber reinforced composite that its use has been ruled unacceptable. Contact with carbon fiber composites increases the corrosion rate of A286 stainless steel a little, and that of 6AL4V titanium not at all, and is acceptable in most cases.
Thus, a need exists for a collar having the strength of an aluminum collar which is corrosion compatible with carbon reinforced composite and has the same or lighter weight.